Excess heat is generated in many electrical and mechanical applications. Microprocessors, for example, may generate excess heat through internal electrical resistances. Mechanical devices such as those employing micro-machined gears and components also generate unwanted heat through friction between contacting surfaces. Heat produced by such systems can severely hamper the performance and reliability of the devices. In an attempt to alleviate the problem of excess heat generation, such devices are often soldered onto a heat sink formed of a thermally conductive material such as copper. Although heat sinks are somewhat effective in removing small amounts of heat, as the amount of heat produced is increased, the finite conductivity and diffusivity of the material used as a heat sink becomes a limiting factor. Even for the most thermally conductive materials such as diamond, the rate of heat transfer away from the device is not fast enough to keep up with the rate at which heat is produced. As a result, unacceptable temperature rises occur thereby limiting the performance and reliability of the device.
Microchannel cooling systems have been introduced which use a liquid such as water for cooling a heat source. In such systems, tunnels or grooves are formed into the substrate supporting the heat generating device. A liquid coolant (typically water) is then pumped through the tunnels in an attempt to remove excess heat generated by the device. These systems are designed to get the liquid coolant very close to the heat source, to further facilitate heat removal. However, the tunnels require a high pressure to maintain sufficient flow of the liquid coolant. As the number of tunnels is increased, and the corresponding size of the tunnels is decreased, the frictional forces on the coolant become substantial and, eventually, prohibitive. Furthermore, the performance of the system is still limited by the finite heat capacity of the water and the ability to get the heated coolant or vapor away from the heat source and fresh coolant into the region.
Heat pipes are also a well known method for providing system cooling. Heat pipe systems utilize a porous material such as sintered metal, ceramic, screens, or webbing as a wick to supply liquid coolant to the area from which excess heat is to be removed. The wick draws the coolant into the desired region through capillary pressure. As the coolant in the wick passes near the heat generating device, excess heat is absorbed through warming or, more commonly, vaporization of the coolant. Unfortunately, wick-type cooling systems also suffer from severe drawbacks. As the coolant in the wick is vaporized, increased pressure is created within the wick. The resultant pressure impedes the flow of new coolant through the wick and into the area where it is most needed. Additionally, the vaporized coolant tends to dry out the wick as it moves through it. Often, as the wick dries out, the temperature of the device rises substantially.
Therefore it is an object of this invention to provide for the efficient dissipation of heat from a heated region having the advantages of conventional microchannel and wick-type systems, but which does not have restricted or impeded flow of liquid coolant, and which does not dry out as quickly due to the flow of vaporized coolant through the system.